The AISI 317L grade is an austenitic stainless steel with high Mo content (3.0 wt% min.). Due to the higher pitting resistance, this grade has replaced AISI 316L steel in many applications where the corrosion resistance is a critical property. However, the high Mo can induce phase transformations in high temperature services. In modern oil refinaries and petrochemical industries AISI 317L has been selected for temperatures as high as 550 °C. The goal of this work was to analyze the microstructural evolution and corrosion resistance of base and weld metal of AISI 317L stainless steel. The welded joints were produced by friction stir welding (FSW). The effect of prolonged exposure at 550 °C was investigated in specimens aged for 200h, 300h and 400h. After each aging treatment microstructural characterization was performed by scanning electron microscopy (SEM). Double loop electrochemical polarization reactivation tests (DL-EPR) were performed to evaluate the degree of sensitization of the samples. The results indicated that the increase of the exposure time at 550 °C promotes the formation of intermetallic phases, which causes corrosion decay of the weld metal.

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The microstructure and mechanical properties of dissimilar butt-joints between a high-strength low alloyed (HSLA) grade and an austenitic high Mn TRIP steel were investigated. The tool rotation and the tool offset toward the TRIP steel were varied between 300–500 rpm and 1–2 mm, respectively. Tool advancing speed amounted to 100 mm/min. Maximum tension stress was observed for the butt-joint welded with 300 rpm and 2 mm offset. The lowest increase in hardness within the stirred zone also occurred for this FSW condition, indicating that this tool rotation is more promising for welding dissimilar joints of commercial HSLA and high Mn TRIP steels. The weld microstructure consisted mainly of a stirred zone, and neither significant HAZ nor TMAZ are observed. However, two main lobular regions are observed, one at the bottom and another one at the top side of the welds. Besides, the HSLA develops a multiphase microstructure consisting of bainite, martensite and retained austenite phases, whereas no e/a martensite is found in the stirred zone of the austenitic high-Mn TRIP steel.

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